Apparatus for tantalum pentoxide moisture barrier in film resistors

ABSTRACT

The present invention discloses a method of manufacturing a thin resistor with a moisture barrier by depositing a metal film layer on a substrate and depositing a layer of tantalum pentoxide film overlaying the metal film layer. The present invention also includes a thin film resistor having a substrate; a metal film layer attached to the substrate; and a tantalum pentoxide layer overlaying the metal film layer, the tantalum pentoxide layer providing a barrier to moisture, the tantalum pentoxide layer not overlaid by and oxidation process.

BACKGROUND OF THE INVENTION

[0001] This invention relates to a method and apparatus for a thin filmresistor having a tantalum pentoxide moisture barrier.

[0002] Current film resistors and the associated processes of makingsuch resistors have had problems with the ability to create or use aneffective moisture barrier. A moisture barrier is that layer that isdeposited on the surface of the resistor in order to prevent moisture inthe form of condensation or vapor from degrading the resistive filmelement. Screen-printed material has been used as a moisture barrier andthis has been shown to reduce the failure rate of the resistor due tomoisture. However, problems remain.

[0003] Tantalum pentoxide has been used in the semiconductor industry asan insulator and to improve recording performance of cobalt alloy mediaon glass-ceramic disks. Tantalum pentoxide has been used within theresistor industry to improve resistive elements integrated with sparkplugs and to form a graze resistor. It is also associated with atantalum nitride resistive system that prevents moisture failure. It isrecognized that tantalum nitride resistors have a naturally occurringlayer of tantalum pentoxide, the result of an oxidation process.Further, tantalum nitride resistors and tantalum nitride capacitors areknown for their resistance to moisture.

[0004] Many thin film resistors, especially those of nickel-chromiumalloys and other alloys containing nickel, chromium, and other metalsare particularly susceptible to moisture conditions. These and othertypes of alloys have a failure mode of electrolytic corrosion that iscapable of causing an electrical open under certain moisture conditions.In particular, under powered moisture conditions, electrolytic corrosioncan occur and the resistor can fail. This makes the thin film resistorunsuitable for applications where moisture conditions may occur.

[0005] Thus, it is a primary object of the present invention to providean improved method and apparatus for a moisture barrier for filmresistors.

[0006] Another object of the present invention is to provide a methodand apparatus for a film resistor which is less susceptible to poweredmoisture testing.

[0007] Another object of the present invention is to provide a methodand apparatus for a moisture barrier capable of use withnickel-chromium, alloy thin film resistors.

[0008] Yet another object of the present invention is to provide amethod and apparatus for a moisture barrier for thin film resistors thatdoes not require tantalum nitride.

[0009] Another object of the present invention is to provide a methodand apparatus for a moisture barrier for a thin film resistor replacesscreen-printed moisture barriers.

[0010] Yet another object of the present invention is to provide amethod and apparatus for a moisture barrier for a thin film resistorthat is compatible with normal manufacturing techniques and materials.

[0011] A further object of the present invention is to provide a methodand apparatus for a moisture barrier for a thin film resistor that canbe used with nickel and chromium alloys.

[0012] Yet another object of the present invention is to provide amethod and apparatus for a moisture barrier for a thin film resistorthat performs favorably under MIL-STD-202 method 103 testing.

[0013] A further object of the present invention is to provide a methodand apparatus for a moisture barrier for a thin film resistor thatperforms favorably under MIL-STD-202 method 106 testing.

[0014] Yet another object of the present invention is to a method andapparatus to reduce or eliminate failures of thin film resistors due toelectrolytic corrosion under powered moisture conditions.

[0015] Another object of the present invention is to provide a methodand apparatus for a moisture barrier that may be deposited throughsputtering.

[0016] These and other objects, features, or advantages of the presentinvention will become apparent from the specification and claims.

BRIEF SUMMARY OF THE INVENTION

[0017] The present invention is a method and apparatus for a tantalumpentoxide moisture barrier in thin film resistors, The inventionprovides for a tantalum pentoxide moisture barrier to be used inmanufacturing a thin film resistor using otherwise standardmanufacturing processes. The invention permits any number of metal filmsto be used as the resistive element. In particular, the inventionpermits nickel-chromium alloys to be used. The resistive metal filmlayer is overlaid with a moisture barrier of tantalum pentoxide. Thetantalum pentoxide layer acts as a moisture barrier.

[0018] The tantalum pentoxide layer results in a thin film resistor thatis resistive to moisture. In particular, the tantalum pentoxide moisturebarrier allows the thin film resistor to be more resistant toelectrolytic corrosion that causes an electrical open under certainmoisture conditions, Thus the present invention provides for increasedreliability in thin film resistors while using substantiallyconventional manufacturing techniques.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a side view of a prior art thin film resistor.

[0020]FIG. 2 is a side view of the thin film resistor having a tantalumpentoxide moisture barrier of the present invention.

[0021]FIG. 3 is a flow chart showing a method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0022]FIG. 1 shows a prior art thin film resistor that may bemanufactured with standard manufacturing processes. In FIG. 1, asubstrate 12 is used. The substrate 12 may be alumina or other substratethat may be used in thin film processes. Overlaid on the substrate is alayer of a metal film which serves as the resistive element for the thinfilm resistor. The metal film layer 14 may be any number of metal filmsbut is often a nickel-chromium (nichrome) alloy or other alloycontaining nickel and/or chromium. Nickel-chromium is one of the mostcommon types of metal films used in thin film resistors. Overlaying themetal film layer 14 is passivation layer 16. The passivation layer 16may be used to protect the thin film resistors electronic propertiesfrom deterioration from external contaminants. The passivation layer 16may be a deposited scratch resistant material such as silicon nitride,silicon dioxide, or other materials such as may be known in the art. Thethin film resistor 10 also includes termination 18. The termination 18on the ends of the thin film resistor is used to electrically connectthe thin film resistor.

[0023] The thin film resistor of the present invention is shown in FIG.2. The thin film resistor 20 is manufactured in a manner similar to thethin film resistor 10 of FIG. 1. However, the thin film resistor 20 ofFIG. 2 also includes a moisture barrier layer 22. The moisture barrierlayer 22 is a layer of tantalum pentoxide film. The tantalum pentoxidefilm may be sputtered onto the thin film resistor, the tantalumpentoxide layer overlaying the resistive metal film layer and optionallya passivation layer. The present invention contemplates that thepassivation layer need not be used.

[0024] The addition of the tantalum pentoxide layer reduces failure dueto electrolytic corrosion that causes an electrical open under certainmoisture conditions. The thin film resistor 20 may use alumina assubstrate 12, or other substrate material. The present invention is noway limited to the particular selection of the substrate, however, thepresent invention is capable of use in standard manufacturing processes.The passivation layer may be a layer of silicon nitride, silicondioxide, or other material such as may be known in the art. The presentinvention contemplates that any number of metal films could be used,including metal films containing nickel, chromium, or both. Termination18 for the thin film resistor 20 may be any type of terminationtypically used with thin film resistors. For example, termination 18 mayinclude wrap around termination.

[0025] The thin film resistor of the present invention using anickel-chromium metal film layer and having a tantalum pentoxidemoisture barrier has been evaluated according to standard environmentaltest methods. The thin film resistor using a 1206-size wrap aroundtermination chip resistor subjected to MIL-STD-202 method 103 tests.These tests are designed to evaluate the properties of materials used inelectronic components as they are influenced by the absorption anddefusion of moisture and moisture vapor. The test is an acceleratedenvironmental test that uses high relative humidity and an elevatedtemperature. According to the test, a temperature of 40° C. and arelative humidity of between 90% and 95% was used, 10 Volts DC wasapplied to the resistors for 96 hours.

[0026] In the 96-hour test, the typical failure rate (without tantalumpentoxide) is from 0 to 4 parts per lot test open. Testing of thetantalum pentoxide moisture barrier thin film resistors where tantalumpentoxide was used as a moisture barrier indicates that there were noopens.

[0027] A second test was conducted with a second group of thin filmresistors having the tantalum pentoxide moisture barrier. For the secondtest, the MIL-STD-202 method 106 was used for testing moistureresistance. This test differs from the previous test as it usestemperature cycling to provide alternate periods of condensation anddrying. According to this test, the temperature range selected wasbetween 65° C. to −10° C. with a relative humidity of between 90% and100%, The test was conducted over a 240 hour period with 10 Volts DCapplied.

[0028] In typical results for the 240 hour test (no tantalum pentoxidemoisture barrier), approximately 90 percent of the resistors test fail.Test results for the 240 hour test where tantalum pentoxide is used as amoisture barrier reveal that there were no failures.

[0029] The method of the thin film resistor of the present invention isbest shown in FIG. 3. The thin film resistor of the present inventioncan be manufactured in a manner substantially consistent with thin filmmanufacturing processes. In step 30 a metal film is deposited throughsputtering or other techniques. The metal film may be of an alloycontaining copper, chromium, nichrome, or other metal such as may beknown in the art. Optionally, in step 32, a passivation layer isdeposited. The passivation layer may deposit through sputtering orthrough other techniques. The passivation layer is used to protect thethin film resistor from external contaminants. In step 34, a layer oftantalum pentoxide is deposited. The tantalum pentoxide layer may bedeposited through sputtering or other techniques. The tantalum pentoxidelayer serves as a moisture barrier to reduce electrolytic corrosion ofthe thin film resistor.

[0030] Thus, an apparatus and method for a thin film resistor having atantalum pentoxide moisture barrier has been disclosed which solvesproblems and deficiencies in the art.

What is claimed is:
 1. A method of manufacturing a thin film resistorwith a moisture barrier comprising: depositing a metal film layer on asubstrate; and depositing a layer of tantalum pentoxide film overlayingthe metal film layer.
 2. The method of claim 1 wherein the step ofdepositing a layer of tantalum pentoxide is sputtering tantalumpentoxide film.
 3. The method of claim 1 wherein the metal film layer isan alloy containing nickel.
 4. The method of claim 1 wherein the metalfilm layer is an alloy containing chromium.
 5. The method of claim 1wherein the metal film layer is a nickel-chromium alloy.
 6. The methodof claim 1 further comprising: depositing a passivation layer overlayingthe metal film layer, the tantalum pentoxide film layer overlaying thepassivation layer.
 7. A thin film resistor comprising: a substrate; ametal film layer attached to the substrate; and a tantalum pentoxidelayer overlaying the metal film layer, the tantalum pentoxide layerproviding a barrier to moisture, the tantalum pentoxide layer not overlayed by an oxidation process.
 8. The thin film resistor of claim 7wherein the metal film layer is an alloy containing nickel.
 9. The thinfilm resistor of claim 7 wherein the metal film layer is an alloycontaining chromium.
 10. The thin film resistor of claim 7 wherein themetal film layer is a nickel-chromium alloy.
 11. The thin film resistorof claim 7 further comprising a passivation layer overlaying the metalfilm layer, the tantalum pentoxide layer overlaying the passivationlayer.
 12. The thin film resistor of claim 7 wherein the tantalumpentoxide layer is overlaid by sputtering.
 13. A nickel-chromium alloythin film resistor comprising: a substrate; a nickel chromium alloy filmlayer attached to the substrate; and a tantalum pentoxide layeroverlaying the nickel-chromium alloy film layer.
 14. The nickel chromiumalloy film resistor of claim 13 further comprising a passivation layeroverlaying the nickel-chromium alloy film layer, the tantalum pentoxidelayer overlaying the passivation layer.